AQA A Level Physics 3.9 Astrophysics (Option A) Exam Questions

About AQA A Level Physics 3.9 Astrophysics (Option A)

Astrophysics is optional topic 3.9 in AQA A Level Physics and is assessed in Section B of Paper 3 if your school chooses it. Section 3.9.1 covers optical telescopes (refracting and Cassegrain reflecting), aberrations, resolving power, CCDs and comparisons with radio, infrared, ultraviolet and X-ray telescopes. Section 3.9.2 covers apparent and absolute magnitude, the parsec, black body radiation, Wien's law, Stefan's law, spectral classes, the Hertzsprung-Russell diagram, stellar evolution, supernovae, neutron stars and black holes. Section 3.9.3 covers the Doppler effect, Hubble's law, the Big Bang, quasars and exoplanet detection. It draws on waves, optics and the photon model from Paper 1.

What you need to know

• 3.9.1 Optical telescopes Know ray diagrams for a refracting telescope in normal adjustment (angular magnification M = fo/fe) and a Cassegrain reflector. Reflectors avoid chromatic aberration and can be made larger; know spherical and chromatic aberration.
• 3.9.1 Resolving power and CCDs Rayleigh criterion: theta is about lambda/D, so larger dishes and shorter wavelengths resolve more detail. CCDs have a much higher quantum efficiency than the eye (typically above 70 percent compared with a few percent), can detect a wider range of wavelengths and record images digitally.
• 3.9.1 Radio and other telescopes Radio telescopes need very large dishes because radio wavelengths are long. Infrared, ultraviolet and X-ray telescopes are usually placed in space because the atmosphere absorbs these wavelengths. Collecting power is proportional to diameter squared.
• 3.9.2 Magnitude and distance The Hipparcos scale: a difference of 1 magnitude is a brightness ratio of about 2.51. Absolute magnitude M is the apparent magnitude at 10 parsecs; m - M = 5 log(d/10). Know the parsec and light year.
• 3.9.2 Black bodies and classification Wien's law lambda(max) T = 2.9 x 10^-3 m K and Stefan's law P = sigma A T^4. Spectral classes O B A F G K M relate to temperature and to Balmer absorption lines of hydrogen. The HR diagram shows the main sequence, giants, supergiants and white dwarfs.
• 3.9.2 Stellar evolution Know the life cycle from main sequence to red giant to white dwarf, or supergiant to supernova to neutron star or black hole. Type Ia supernovae are standard candles. The Schwarzschild radius is Rs = 2GM/c^2.
• 3.9.3 Cosmology Doppler shift z = delta f / f = v/c for v much less than c. Hubble's law v = Hd gives an estimate of the age of the universe of about 1/H. Evidence for the Big Bang includes cosmological red shift, the cosmic microwave background and the abundance of hydrogen and helium. Exoplanets are detected by the radial velocity and transit methods.

Common mistakes in 3.9 Astrophysics (Option A)

• Getting the magnitude scale backwards. A smaller (or more negative) magnitude means a brighter star.
• Using the diameter of the eyepiece for resolving power. Resolution depends on the diameter of the objective lens or mirror.
• Saying that stars on the main sequence change position as they age. A star stays at roughly the same point while fusing hydrogen, then leaves the main sequence.
• Forgetting to convert Hubble's constant into s-1 before finding the age of the universe. Convert km s-1 Mpc-1 into SI units first.
• Stating that red shift is caused by stars moving through space only. Cosmological red shift is due to the expansion of space itself.

Try three 3.9 Astrophysics (Option A) exam-style questions